This invention relates to the separation of organic compounds having closely related boiling points which as a result are difficult to separate. More particularly it relates to an established process for separating such close boiling compounds, i.e., extractive distillation. Even more particularly the invention relates to furfural extractive distillations.
Basically extractive distillation involves distilling the mixture to be separated in the presence of a relatively higher boiling solvent which is selective for a component of the mixture. Extractive distillations are in general performed by causing the solvent to flow down the distillation column as the distillation proceeds, and as vapors ascend from the kettle at the bottom of the column. Thus the vapors are scrubbed by the solvent in a first zone, which selectively dissolves the more soluble component and the resulting fat solvent is partially stripped in a second zone by vapor coming from the distillation kettle and having a higher temperature than the vapor in the first zone. As in ordinary distillation a portion of the overhead product is returned to the distillation column after condensation as reflux, the reflux inlet being positioned several plates above the solvent feed entry. The component dissolved by the solvent may be removed from the solvent by distillation or any other suitable means in order to produce a lean solvent which may be recycled to the extractive distillation.
Extractive distillation is used extensively in the commercial separation of hydrocarbons having 4 to 5 carbons, more particularly the analogues of a particular class of hydrocarbons in this range. For example, C.sub.4 hydrocarbon analogues comprising one or more of butadiene, butenes and butanes. The more unsaturated compound in the mixture is more soluble in the solvent, hence the solvent is selective for the more unsaturated compound of the mixture being separated. Thus in the example given, the normal extractive distillation selectively removes butadiene from the vapors having a mixture of butadiene, butenes and butanes. In a similar manner the C.sub.5 hydrocarbons are selectively separated. The same selective solubility relationship applies between the butenes and butane if these are the components of the mixture with the butenes being selectively dissolved in the solvent.
The sources of the C.sub.4 -C.sub.5 hydrocarbons are quite diverse such as oxidative dehydrogenation, catalytic or thermal cracking, Fisher-Tropsch reactions and other sources well known to the art. Generally in addition to the hydrocarbons, there are other condensable and non-condensable gases in the feed streams. Thus, the extractive distillation is usually one portion or segment of an overall refining operation which is directed to obtaining one or more of the hydrocarbon components of the feed as product.
There are a number of variations in process equipment and solvent systems proposed in the art and in existence. Foremost among the solvents disclosed in the art and employed in commercial operations is furfural. Furfural may be so prevalent because of its availability and low cost as well as its excellent absorptive separative properties. In any event a large number of existing operations are designed and operated for furfural solvents, similarly a vast amount of technical information has been accumulated in regard to furfural systems. The principal effort of the art in regard to furfural solvent systems has been to improve the efficiency of the system by modification of the manner of operation and not a great deal of success has been achieved in modifying the solvent system to improve the operation of the system. One widely recognized modification of furfural is the addition of minor amounts of polar, lower boiling materials to the furfural, such as water, methanol, ethanol, propanol, acetaldehyde, acetone, methyl ethyl ketone, ether, propyl ether, ethylene dichloride, ethyl acetate, methyl formate, and the like. Water and similar materials are not cosolvents and serve other purposes in the process, principally in regard to providing lower temperatures for stripping solublized hydrocarbon out of the furfural and improving the preferential selectivity of furfural.
A particular problem encountered in adding cosolvents or non-solvents to the furfural system is a possible detriment to solvent selectivity and capacity. It has been found that the addition of most materials, even other solvents to the furfural acts in the same manner as the solvation of hydrocarbon therein, hence reducing the solvent capacity of the furfural.
Another problem relating to the use of cosolvents is the potential for reaction between furfural and the proposed cosolvent. For example, U.S. Pat. No. 2,366,360 to Semon, discloses a number of selective solvents for butadiene such as nitrobutane, nitrobenzene, aniline, dichlorodiethyl ether, ethylene chlorhydrin, dioxane, crotonaldehyde, alpha-ethyl hexanol, cyclohexanone, acetaphenone, mesityl oxide, diethyl oxalate and the like which may be substituted for furfural. The unknowing may misinterpret this teaching to mean that these materials may be partially substituted for furfural. To so misinterpret Semon's teaching can be hazardous. The first substitute disclosed by Semon, i.e., nitrobutane, reacts with carbonyl compounds, such as furfural, to yield unstable nitro alcohols, H. B. Hass and E. F. Riley, Chem. Rev. 32, 373 - 430 (1943). The second member of Semon's list, nitrobenzene exhibits similar properties. The third member of the Semon listing, aniline is also reactive with furfural, at room temperatures to yield anils (Schiffs bases).
It is an object of this invention to provide a furfural cosolvent system for extractive distillation which is more efficient than the corresponding furfural system. Another object is to provide a furfural-cosolvent system with no diminution of selectivity for the more unsaturated hydrocarbon than the corresponding furfural system. Another object is to provide a furfural-cosolvent system which can be operable in present furfural equipment. An additional object is to provide a furfural-cosolvent system which will tolerate water in about the same proportions as furfural alone. These and other objects and advantages of the present invention will be apparent from the following discussion.